In wireless communications networks, such as those compliant to the IEEE 802.16 technical standard, a base station (BS) may communicate with one or more subscriber stations (SS) over a downlink (DL), while the SS may communicate with the BS over an uplink (UL). An SS makes use of a contention channel in the UL connecting the SS with the BS to transmit requests to the BS for ranging and/or bandwidth allocation purposes. Ranging requests may be used by the SS to obtain access to the BS and may include several different types of requests, including a) an initial ranging that may be used for network entry, b) periodic ranging that may be used for network synchronization, and c) hand-off ranging that may be used to acquire a new member into a diversity set of the SS. Bandwidth allocation requests may be used to request bandwidth specifically dedicated to the SS to allow the SS to transmit data. If the BS grants the SS's request, the SS may be allocated a contention free transmission period to perform its requested operation.
In an IEEE 802.16 compliant wireless communications network, orthogonal frequency division multiple access (OFDMA) may be used to make efficient use of available transmission bandwidth as well as allowing multiple transmissions to occur simultaneously. Transmissions using OFDMA may take place over two different dimensions, a time-frequency dimension and a code dimension. The SS, desiring to transmit a request to the BS, may randomly select a time-frequency resource that has been allocated as being a part of a contention channel and a code out of a set of possible codes to encode its transmission. The transmission of the SS, after being modulated to the selected frequency and encoded by the selected code, may be transmitted to the BS at the selected time.
FIG. 1a illustrates a structure of a frame 100 of an IEEE 802.16 compliant wireless communications network operating in a time division duplex (TDD) mode. The frame 100 includes a downlink (DL) subframe 105 that may be used for transmissions originating at the BS and an uplink (UL) subframe 110 that may be used for transmissions originating at the SS. Since the wireless communications network is operating in TDD mode, transmissions made in the DL subframe 105 and the UL subframe 110 utilize the same frequency range and are separated in time.
The DL subframe 105 includes a preamble 115 that may be used for PHY layer synchronization, a frame control header (FCH) 117 that may be used for specifying a profile and length of one or more DL bursts, such as DL burst #1 119, immediately following the FCH 117. The UL subframe 110 includes a contention slot for use in initial ranging 120, a contention slot for bandwidth requests 122, and one or more UL protocol data units (PDU) from the SS, such as a UL PDU from SS #1 124. A UL PDU includes a preamble 126 and a UL transmission burst 128.
FIG. 1b illustrates a structure of a frame 150 of an IEEE 802.16 compliant wireless communications network operating in a frequency division duplex (FDD) mode. The frame 150 includes a DL subframe 155 and a UL subframe 160. Since the wireless communications network is operating in FDD mode, transmissions made in the DL subframe 155 and the UL subframe 160 may utilize different frequency ranges and may occur during the same time.
The DL subframe 155 includes a preamble 165 that may be used for PHY layer synchronization, an FCH 167 that may be used for specifying a profile and length of one or more DL bursts, such as DL burst #1 169, immediately following the FCH 167. The UL subframe 160 includes a contention slot for use in initial ranging 170, a contention slot for bandwidth requests 172, and one or more UL protocol data units (PDU) from the SS, such as a UL PDU from SS #1 174. A UL PDU includes a preamble 176 and a UL transmission burst 178.
In an IEEE 802.16 compliant wireless communications network, the amount of uplink resources allocated to the contention channel may be chosen by the BS and then signaled to the SS in the wireless communications network in a UL channel descriptor (UCD) message. The BS may change the uplink resource allocation as it sees fit, needing only to inform the SS in the wireless communications network of any changes in the uplink resource allocation.
The amount of uplink resources dedicated to the contention channel (for example, a number of channels or sub-channels of the uplink dedicated solely for the contention channel) for the purpose of ranging and/or bandwidth requests may have an impact on the overall performance of the wireless communications network. For example, if a small amount of resources is allocated to the contention channel, then more resources may be freed for the actual transmission of data, which may increase the data throughput. However, with relatively few contention resources, transmission collisions of the ranging and/or bandwidth requests may occur with relatively high frequency. This may lead to the SS experiencing long delays in waiting for their ranging and/or bandwidth requests to be granted, thereby reducing the overall performance of the wireless communications network. Alternatively, if a large amount of resources are allocated to the contention channel, then the SS may experience short delays in their ranging and/or bandwidth requests. However, since few resources are available for the actual transmission of data, the overall data throughput of the wireless communications network may be low, resulting in a slow wireless communications network.
One commonly used technique for determining the amount of uplink resources to be dedicated to the contention channel may be through the analysis of actual network traffic and/or simulation studies. For example, collected network traffic and/or simulation studies for a variety of application types and loads may be used to determine the amount of uplink resources to be dedicated to the contention channel. The results of the analysis (such as a table of uplink resources to be allocated for the contention channel indexed by traffic type and load) may be stored in the BS and then depending on expected traffic type and load, the BS may select the amount of uplink resources to be allocated for the contention channel and then broadcast the uplink resource allocation to the SS using a UCD message.